1. Field of the Invention
The present invention relates generally to a method of and apparatus for shaping a light beam within an optical scanning system. More specifically, the invention relates to the shaping of a laser beam into a non-Gaussian configuration. The invention further extends to an indicia or bar code reader incorporating such apparatus.
2. Description of the Related Art
Optical scanners, such as bar code symbol readers, are now quite common. Typically, a bar code symbol comprises one or more rows of light and dark regions, typically in the form of rectangles. The widths of the dark regions (bars) and/or the widths of the light regions (spaces) between the bars represent encoded information. A bar code symbol reader typically illuminates the symbol with a scanning laser beam, and the widths of the bars and/or spaces are deduced from the reflected light. This data can then be used to decode the bar code symbol and to recover the underlying high-level information which it represents.
The ease with which the bars and spaces can be distinguished, and their widths determined, depends upon very many factors, an important one of which is the characteristics of the laser beam xe2x80x9cspotxe2x80x9d which is formed when the laser beam falls onto the symbol. Since the symbol is generally scanned in a direction perpendicular to the beam axis, the beam xe2x80x9cspotxe2x80x9d corresponds with the beam cross-section at that particular point. With the use of conventional shaping optics the power density in the beam cross-section is generally Gaussian in form, although possibly with a different spread in the x-direction (the direction of scanning) and in the y-direction(perpendicular to the direction of scanning). Where the x and y spreads are not the same, the beam has an elongated or quasi-eliptical cross-section.
One of the requirements, for effective scanning is an adequate working range, in other words the range of distances from the scanner that the bar code symbol may be placed and still be decoded. It is typically only over a limited working range that the cross-section of the beam takes up an appropriate generally elongated shape to allow reliable decoding. This is particularly evident where the beam is provided by means of a laser diode, since these exhibit substantial astigmatism. Typically, the application of shaping optics applied to such a beam creates a scanning beam in which the cross-section varies with distance from the scanner. The point at which the x-dimension of the beam cross-section is narrowest (the xe2x80x9cx-waistxe2x80x9d) may be at a different distance from that at which the y dimension is narrowest (the xe2x80x9cy-waistxe2x80x9d).
It is well known that Gaussian beams, when used for scanning a target, provide a signal with excellent contrast. However, Gaussian beams suffer from limited working ranges, and the smaller the waists are designed to be the shorter the working ranges become. Since the working ranges are effectively those where the beam cross-sections remain essentially constant, these correspond generally with the Rayleigh ranges.
An alternative approach is to use the beam-shaping devices known as axicons: see U.S. Pat. Nos. 5,331,143 and 5,164,584, both commonly assigned with the present application. A related approach is disclosed in U.S. Pat. No. 5,315,095. Further theoretical and practical background information may be found in the following papers: J. E Dumin, xe2x80x9cExact solution for non-diffracting beams. I. The scalar theory,xe2x80x9d JOSA A4, 651-654 (1987); J E Dumin, J J Meceli and J H Eberly, xe2x80x9cDiffraction-free beam,xe2x80x9d Phys. Rev. Lett. 54, 1499-1501 (1987); and E Marom and J Katz, xe2x80x9cScanning with axicon-generated beams,xe2x80x9d OSA 1992 Annual Meeting, New Mexico, WW4.
It is one object of the present invention to provide a simple, relatively inexpensive, means of creating a non-Gaussian beam, for example a laser beam to be used in an optical scanner.
It is a further object to provide a beam generator, or beam shaping system, and an optical scanner, in which some of the difficulties associated with the use of Gaussian beams are eliminated or alleviated.
It is a further object of the present invention to provide a means of increasing the working range of an optical scanner. A related object is to provide a convenient means of controlling the working range of the beam.
According to the present invention there is provided a beam generator, or beam shaping system, comprising a laser for producing an initial laser beam, a beam shaping element for shaping the initial laser beam, and a phase filter for altering the wavefront of the shaped beam to create a non-Gaussian outgoing beam. The invention also extends to an optical scanner, for example a hand-held or portable bar code scanner, incorporating a beam generator of this type.
Preferably, the beam shaping element may comprise a diffractive optical element, a holographic optical element, or an anamorphic lens. The phase filter may also comprise a diffractive optical element or a holographic optical element. The filter preferably takes the form of a Bessel filter (i.e. a filter that converts an incoming plane wave to an outgoing wave having a power density that varies as the Bessel function J0). The filter may for example comprise a plurality of concentric rings. The phase filter could either comprise a phase-only filter, or alternatively a combined phase and amplitude filter. The phase filter could also comprise an etched substrate.
In the preferred embodiment, the outgoing beam has a power density which comprises a Bessel-Gaussian beam, or a coherent combination of a Bessel beam and a Gaussian beam, which does not vary exclusively according to the Bessel Function J0.
In a preferred embodiment an anisotropic beam is synthesized using a phase-only filter illuminated by a collimated laser diode beam. When using a laser diode as a source, the beam leaves the collimation optics as a Gaussian beam. It then passes through an aperture to produce a truncated Gaussian beam. The truncated Gaussian beam is filtered through a Bessel filter to reduce the edge effects introduced by the aperture. The beam waist of the truncated Gaussian beam is matched to the size of the center circle of the Bessel filter, thereby allowing a high percentage of the beam""s power to be transmitted through the center circle of the phase filter while rendering negligible any edge effects or high spatial frequency noise, such as rippling or the like, introduced by the truncation of the beam by the aperture.
Extended confinement range is obtained at the expense of reduced contrast for a wide range of spatial frequencies. Substantial improvements in the range of the beam can be achieved depending on the optimized spatial frequency and the desired contrast level.
The present invention, in one or more of its various forms, supplies a number of distinct advantages over the conventional axicons used in the prior art. More specifically, the tilt angle may be almost twice as large as that for a conventional axicon. In addition, the longer spacing between the collimation lens and the phase grating than that used for a conventional axicon avoids the boundary-diffraction/irregular-reflection from the rim of the aperture. In the preferred embodiment, the use of a phase grating allows the zero-order mode to propagate together with the first-order mode; these are equivalent, respectively, to a Bessel-type of non-diffracting beam and a truncated Gaussian type diffracting beam. The non-diffracting beam has an extended working range of higher density bar code symbols, whereas the diffracting beam response deals with lower density symbols for distant decoding. Accordingly, the outgoing beam may comprise a coherent combination of a diffracting beam and a non-diffracting beam.
The preferred phase grating has a grating constant of 40 micrometers. The zero-order is strong, with measurements showing that between 30% and 40% of the total optical power goes into the zero order.
The coherent combination of two beams, described in the various embodiments of the present invention, allow the user to make use of the beams different propagation properties in a very simple and convenient way, for example for bar code data capture.
The novel features which are considered characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings.